Many electronic circuit applications require large impedances with high levels of precision (e.g., 1% or better). However, large precision resistors having values of resistance greater than 1 M.OMEGA. are not standard and very costly to obtain. Likewise, capacitors in the 100 .mu.F range are very expensive and are typically only precise to 10%. In this range, most capacitors must be electrolytic which introduces problems of leakage and limits the environment in which they may be used. Similarly, inductors with large inductances are heavy and expensive. These large value resistors, capacitors, and inductors are bulky as well as expensive, which exacts a hidden cost in terms of circuit real estate.
An example of an application requiting large, highly precise resistances is the closed loop simulation of mechanical elements under electronic control. Utilizing actual mechanical hardware during development of the electronic controls is often costly and possibly dangerous until the stability of the electronic control has been proven. To reduce the cost and increase the safety and efficiency of the controls development stage of a program, the mechanical hardware itself may be electronically simulated through the use of low pass filters with extremely low corner frequencies.
Many of these mechanical elements, such as servo valves for frequency regulation of aircraft-mounted electric power generators, require these electronic filters to have corner frequencies of only a few millihertz. The RC product must be large to produce a low corner frequency. A large resistance is undesirable because of the cost associated with the precision tolerances required, the bias-current constants on active devices, and the noise-floor limits. Large capacitors simply are not available in tight tolerances, are physically large, and prone to leakage. As a result, the design of such a filter would typically require the use of resistors in the 20-30 M.OMEGA. range. Due to the sensitive nature of electronic controls development, however, the precision of the simulation is critical, and resistors having 1% precision or better are often required. Specifically, corner frequencies of less than 22 mHz may be required to accurately simulate the response of such a servo valve.
Other applications requiring the use of very large, highly accurate resistors to construct low pass filters having corner frequencies in the millihertz range include variable speed constant frequency (VSCF) power conversion systems. In these systems, which typically comprise a variable frequency engine driven generator coupled through a constant frequency electronic power converter to the utilization equipment, a failure in the electronic power conversion stage may result in unwanted DC content in the output waveform. To monitor for and protect against such occurrence, the converter controls must filter out all but the pure DC content of the output waveform. The use of a filter with a corner frequency in the millihertz range ensures that the system is not taken off line unless the output waveform actually contains a precise mount of DC content.
Typically, the construction of these extremely-low pass filters and circuits require that very expensive and bulky precision resistors and large values of capacitance be used. Along with their large size, these large precision resistors and capacitors carry a large price which tends to drive up the total cost of a design. Making room on a circuit board to accommodate these large resistors and capacitors also has a certain cost impact as described above, especially when more or larger boards become necessary. Additionally, the size of the housing may be forced to increase. Large variable precision resistors are also needed in many applications where a dynamic response or dynamic control is required. However, large precise variable resistors or potentiometers are not commonly available, and those that are custom made are relatively expensive.
U.S. Pat. No. 5,124,586, granted Jun. 23, 1992 to Carobolante for an Impedance Multiplier presents one method of multiplying the input resistance of a circuit for use in the construction of integrated circuits. To allow for resistance multiplication for both positive and negative circuit voltages, the circuit of Carobolante (illustrated in FIG. 1) utilizes a complex circuit comprising four transistors, six resistors, and two diodes coupled to a separate voltage supply. This circuit, however, possesses an admired degree of non-linearity which makes it unusable in applications such as those described above. Furthermore, the equivalent circuit of Carobolante is a resistor in series with a dc voltage source, which introduces an unacceptable dc bias requirement. This bias-up aspect of the Carobolante circuit totally precludes its applicability in VSCF applications described above. Additionally, the circuit of Carobolante will not operate to multiply capacitive impedances at all. As such, this circuit is unacceptable for use in applications which require a large value of capacitance, such as timing circuits.
It is a goal, therefore, of the instant invention to overcome these and other problems associated with the use of large value, precision impedances, such as resistors, capacitors, and inductors. Specifically, it is a goal of the instant invention to provide a means of multiplying either the resistance of a resistor, the capacitance of a capacitor, or the inductance of an inductor by either fixed or variable multiplication factors. Further, it is a goal of the instant invention to provide an extremely-low pass filter using readily-available, reasonably-priced, and reasonably-sized components. Also, it is a goal of the instant invention to provide a voltage controlled variable impedance of a large value without inducing unacceptable distortion levels in a processed signal. It is also a goal of the instant invention to improve the linearity and performance of circuits requiring extremely large impedance values.